Smoke reducing method and apparatus for stokers



July 17, 1962 E. R. MITCHELL ETAL SMOKE REDUCING METHOD AND APPARATUS FOR STOKERS Filed July 11, 1960 1 Tom $12M 2644M I ATTORNEY United States Patent 3,044,422 SMOKE REDUCING METHOD AND APPARATUS FOR STOKERS Earland R. Mitchell and Frank D. Friedrich, Ottawa, 0n-

tario, Canada, assignors to Her Majesty the Queen in right of Canada as represented by the Minister of Mines and Technical Surveys Filedjuly 11, 1960, Ser. No. 42,094 2 Claims. (Cl. 110-72) This invention relates to a method and apparatus for reducing smoke in the operation of underfeed stokers.

As is well recognized, an excessive amount of smoke is produced during the operation of-conventional commercial or small industrial stokers particularly in the burning of bituminous coals of low ash fusion type. Such stokers are operated in an on-01f fashion by simple well known controls. Combustion experiments have shown that the excessive smoke is caused by this on-0E operation of the stoker.

In most stokers, the worm and fan are driven by the same motor and, indeed, it is difilcult or impossible to employ separate motors. Thus, when the stoker has satisfied the demand for heat the coal feed must be stopped and in doing so the air fan stops also. When the combustion air is shutoff the fire bed immediately begins to cool and loss of ignition of the fuel occurs rather rapidly. During the early stages of loss of ignition, smoke is produced because there is not enough air to burn the gases produced by the residual heat of the fuel bed.

When the boiler or heat-ing device demands more heat, the stoker coal feed is turned on. At the same time the air fan supplies a full blast of air to the cool fire bed which has lost ignition to an extent depending upon the length of time the stoker was off. The blast of air reignites the fuel bed rapidly and this process of rapid ignition" is comparable to kindling a fresh fire since it produces excessive smoke. A

It is an object of this invention to provide a simple and effective method and apparatus for eifecting substantial reduction of smoke in the operation of underfeed stokers.

The invention will be described with reference to the accompanying drawing, the single FIGURE of which illustrates diagrammatically in side elevation an apparatus in accordance with the invention.

The conventional equipment shown includes a stoker 1 associated with a boiler or the like having a front wall 2 and a bridge wall 3. The stoker has a retort 4 fed by -.a worm 5 from hopper 6 and driven by motor 7. The

usual stoker fan is indicated at 8 and has a duct 9 leading to windbox 10. The fuel or fire bed is indicated at 11. The high pressure overfire turbulence fan is shown at 12 and it has a main duct 13 with a branch 14 communicating with overfire nozzles 15. Duct 9 has a damper 16 which may be of standard type and arranged to open in response to a flow of air produced by operation of the fan 8 and to close automatically when the fan ceases to operate.

In accordance with the invention, a branch duct 17 leading from the main fan duct 13 provides communication between such duct 13 and the windbox 10. A key damper 18 of conventional type is provided in branch duct 17 and a similar damper 19 is provided in branch duct 14.

As shown, a damper motor 20 is provided to operate the dampers 17 and 18. An arm 21 is fixed to each damper shaft 22 and 23 and such arms are connected by a link 24. The damper motor shaft 25 is connected to an arm 21 by a link 26. A spring 27 holds the dampers in a position in which damper 19 is closed and 3,044,422 Patented July 17, 1962 damper 18 is open. Thus, on energization of the motor 20, the latter operates to open damper 19 and close damper 18 and to hold the dampers in such positions so long as the motor is energized. The damper motor 20 and the damper operating structure are of conventional t pe.

The stoker motor starter switch 28 is electric-ally connected in parallel with the damper motor 20 as by a circuit 29 wherebyenergization of the stoker motor.(a's during its on period) will result in energization of the damper motor 20 to close damper 18 and open damper 19 whereas de-energization of the stoker motor (as during its off period) will result in opening of damper 18 and closing of damper 19.

Preferably, the inlet of overfire fan 12 is located by means of a duct 30 at as great a height as possible in the room in which the equipment is installed in order to utilize the ambient heat available in such room and also to assist in ventilation of the room. It will also be apparent that, if necessary or desirable, the inlet air for fan 12 may be preheated in any suitable manner as by means of a heat exchanger.

It will be apparent that, in operation, during the ofi periods of the stoker, a supply of air under pressure will be fed to the windbox through duct 17 by means of the overfire fan 12. This supply of air, although very much below the capacity of the stoker fan 8, will be suflicient to maintain a degree of ignition in the fuel bed such that, when the stoker fan comes into operation, substantially no smoking of the fuel bed will occur. Generally speaking, the fan 12 will have a capacity of the order of 10% that of the stoker fan 8 and will supply air at 25 to 30 in water pressure. However, it will be understood that the capacity of the fan 12 may be increased if required to, say, 25% of the stoker fan capacity.

Under certain circumstances, it may be necessary to provide an operating motor 3-1 for the damper 16 in the stoker fan duct 9. This motor 31 may also be connected, as indicated at 32 to the circuit 29. It will be understood that the damper 16 will normally be held in a closed position but on energization of the stoker motor, its motor 31 will also be energized to open the damper.

.We claim:

1.. A method of operating a stoker having a fuel receiv ing bed, a windbox, a stoker fan having a duct for sup ply of air to said windbox, a motor operable to drive said stoker fan and having an on-olf cycle and an overfire fan having a first duct leading to the space above said bed and a second duct leading to said windbox, which comprises the steps of, continuously supplying air under pressure from said overfire fan to said overfire fan ducts and during the oif portion of said cycle, directing a supply of air under pressure from said overfire fan to said windbox through said overfire fan second duct while closing said stoker fan duct and overfire fan first duct and, during the on portion of said cycle, interrupting said supply of air from said overfire fan second duct to said windbox while opening said first duct and stoker fan duct, and directing a supply of air under pressure from said stoker fan through said stoker fan duct to said windbox, said windbox thereby constantly receiving a supply of air under pressure.

2. In a stoker having a fuel receiving bed, a windbox beneath said bed, a stoker fan having an air supply duct leading therefrom to said windbox, a, motor operable to drive said stoker fan, a switch controlling operation of said motor and actuable to energize said motor to supply air under pressure from said fan through said duct to said windbox and to de-energize said motor to arrest the flow of air under pressure through said duct, and an overfire fan having a main air supply duct in constant communication with said overfire fan and thereby arranged to contain a constant supply of air under pressure, a stoker operating apparatus comprising a first air supply branch duct leading from said overfire fan duct to a space above said bed, a second air supply branch duct leading from said overfire fan duct to said windbox, a damper in each of said branch ducts 1dr controlling the supply of air flowing therethrough, means responsive to de-energization of said motor to open said second branch damper for supply of air under pressure from said-overfire fan through said second branch duct to said Windbox and to close said first branch duct'fto arrest the supply of air under bed, andmeans'responsive pressure from said overfire ian to said space above said to energizat-ion of said motor to close said second branch damper and to open said first branch damper, 'saidwindbox thereby receiving a constant supply of air under pressure.

References Cited in the file of this patent 1 UNITED STATES PATENTS 

